This invention relates to interactive communication within smoke and fire detection systems.
In detection systems in which a central controller is connected to a number of transponders (containing smoke/fire detectors) by a two-wire communication line, it is known to both supply power to the transponders and pass data to and from the transponders over the line. The controller periodically "polls" and tests the status (e.g. the smoke sensitivity) of a selected transponder by addressing the selected transponder, over the communication line, and sending a command for the transponder to return a signal representative of its status to the controller via the line.
In one such system, described in Vogt, et al., U.S. Pat. No. 4,507,652, "Bidirectional Interactive Fire Detection System," issued Mar. 26, 1985, the controller selectively addresses the transponders by transmitting a succession of pulse groups over the line, each pulse group corresponding to one of the transponders. Each transponder counts the number of pulse groups until its pulse group has been reached and then responds to and command included in that pulse group. The commands are indicated by extending the duration of particular pulses in the group. The transponder senses when a pulse has remained high beyond a threshold time duration to detect the elongated pulse and hence the corresponding command. The transponder transmits sensitivity data to the controller by drawing a current pulse from the communication line having a duration corresponding to the sensitivity of the detector. The controller measures the current pulse duration to determine whether the detector sensitivity is in, for example, a "normal" or "alarm" range. The transponder also is able to return a separate calibration signal to the controller to more accurately determine the status (i.e. "normal" or "alarm") of the detector.